If one auxiliary light is out, what is the maximum speed a locomotive can go?

The correct choice is based on the regulations governing auxiliary lights on locomotives. When a single auxiliary light is out, the locomotive is generally permitted to operate at the maximum authorized speed, as long as the remaining lighting is functioning properly and compliant with safety regulations.

Auxiliary lights are important for visibility and safety, particularly in low-light conditions; however, the loss of one auxiliary light does not necessarily compromise the entire operation of the locomotive. Therefore, as long as the locomotive is otherwise in good working condition and meets all safety requirements, it can travel at its maximum authorized speed.

In contrast, the other options specify lower speed limits that may be overly restrictive given the situation. The regulation allows for maintaining higher operational speeds to ensure efficiency of service, provided safety standards are met even with one auxiliary light inoperative.